Three-stage niobium mineralization at Bayan Obo, China

ABSTRACT The Chinese Bayan Obo deposit is a world-class rare earth element (REE) deposit with considerable niobium (Nb) and iron (Fe) resources. A complete genetic understanding on all metals is fundamental for establishing genetic models at Bayan Obo. With extensive research being focused on REE enrichment, the timing and controls of Nb enrichment remain unresolved at Bayan Obo, which is mainly due to the challenges in dating, i.e. multistage thermal events, fine-grained minerals with complex textures and the rare occurrence of uranium-enriched minerals with mature dating methods. Based on robust geological and petrographic frameworks, here we conducted ion probe uranium-lead (U-Pb) dating of ferrocolumbite to unravel the timing, hence the genesis of Nb mineralization. Three types of hydrothermal ferrocolumbites—key Nb-bearing minerals—are identified based on their textures and mineral assemblages. They yield U-Pb ages of 1312 ± 47 Ma (n = 99), 438 ± 7 Ma (n = 93), and 268 ± 5 Ma (n = 19), respectively. In line with deposit geology, we tentatively link the first, second and third stage Nb mineralization to Mesoproterozoic carbonatite magmatism, ubiquitous early Paleozoic hydrothermal activity, and Permian granitic magmatism, respectively. While quantifying the contribution of metal endowment from each stage requires further investigation, our new dates highlight that multi-stage mineralization is critical for Nb enrichment at Bayan Obo, which may also have implications for the enrichment mechanism of Nb in REE deposits in general.


INTRODUCTION
Carbonatite-related mineralization is the world's primary source of rare earth element (REE) and niobium (Nb), with the overall metal endowment being controlled by a few deposits.For example, the Araxá and Catalão-II (Brazil) and St. Honoré (Canada) deposits account for ∼98% of annual global Nb production, while the Bayan Obo (China) deposit accounts for ∼40% of annual global REE production [1 ].Despite their economic importance and extensive studies, processes controlling metal enrichment in these giant systems remain controversial.The Bayan Obo deposit, renowned for its vast REE resources and substantial Nb and iron (Fe) reserves, is a classic example for this puzzle.
Existing dates of Nb mineralization are scarce, based on the occurrence of pyrochlore [(Ca, Na) 2 Nb 2 O 6 (OH, F)] from skarn in contact with Permian granites [5 ], and a Th-Pb isochron date at ∼273 Ma ( n = 7;) for aeschynites [Ce(Ti, Nb) 2 O 6 ] [11 ], a Permian stage Nb mineralization was suggested.In contrast, aeschynites in vein-type ores yielded disparate results from 658 ± 36 Ma (Sm-Nd isochron, n = 5; [12 ]) to 438 ± 25 Ma (Th-Pb isochron, n = 4; [5 ]), to 290 ± 15 Ma (Rb-Sr isochron, n = 4; [12 ]).Variability in these dates from the same ore type may result from isotopic system disturbance [12 ].Deciphering the genesis of Bayan Obo relies on faithful interpretation of radiometric dates.The REE-and Th-rich nature of minerals like monazite and bastnäsite has prompted the widespread use of Sm-Nd and Th-Pb dating.However, the limited range of Sm/Nd ratios poses challenges for accurate and precise isochron dating, evident in substantial errors tied to errorchron [13 ].Meanwhile, Th-Pb dating has yielded dates ranging from ∼1.2 to 0.26 Ga [9 ].Given the common occurrence of galena and the inability of common lead corrections using LA-ICP-MS, the variations in Th-Pb dating could be partly explained by varying proportions of common lead.Additionally, this variability may also be linked to multiple thermal events, which caused the formation of multistage minerals and open system behavior of the Th-Pb system [14 ].
High-quality radiometric dating is essential to advance our understanding on the genesis of Bayan Obo.Here we approach this challenge from Nb mineralization and focus on dating ferrocolumbite [(Fe, Mn) (Nb, Ti, Ta) 2 O 6 ].As a major Nb-bearing mineral at Bayan Obo [15 ,16 ], ferrocolumbite constitutes ∼90% Nb in the dolomite host rock [17 ].They incorporate significant amounts of U (up to hundreds of ppm), and leverage the dual U decay system ( 238 U-206 Pb and 235 U-207 Pb) as a powerful tool for dating complex geological systems and evaluating closed-system behavior.
Here we conducted a high-spatial-resolution ( ∼10 × 15 μm 2 ) secondary ion mass spectroscopy (SIMS) ferrocolumbite U-Pb dating approach using a matrix-effect correction strategy [18 ].We successfully obtained the first set of U-Pb ages for Nb mineralization at Bayan Obo, which were used to yield implications for deposit genesis.

DEPOSIT GEOLOGY AND SAMPLES
The Bayan Obo deposit is hosted primarily by a dolomitic unit within the Mesoproterozoic Bayan Obo Group (H1-H9 units; Fig. 1 A; [5 ]).The dolomitic unit was initially referred as 'H8 dolomite' [5 ], which is now considered to be carbonatite [19 -23 ].Three crucial tectono-thermal events influenced the Bayan Obo deposit, including Mesoproterozoic carbonatite magmatism, early Paleozoic hydrothermal alteration event, and Permian granitic magmatism.Abundant carbonatite dykes adjacent to the deposit [8 ,24 ], of which a few have been dated at ∼1.4-1.2Ga by zircon U-Th-Pb dating [7 ,8 ], pointing to a Mesoproterozoic carbonatite magmatism.Undeformed to weakly deformed veinlets are commonly observed cutting across both the orebody and the H8 dolomite [25 ].These veinlets exhibit diverse mineral compositions, encompassing REE-and Nb-bearing minerals, along with gangue minerals like fluorite, aegirine, alkaline amphibole, mica, pyrite, baryte, and molybdenite.Various dating results, including Sm-Nd, Rb-Sr, Re-Os isochron ages and Th-Pb dates, suggest that these veins were formed at ∼0.4-0.5 Ga [21 ,25 -27 ].Extensive Permian granites border the deposit to the south and east, leading to extensive hydrothermal metasomatism at the contact zone between granites and H8 dolomite in the east mining area-the so called East Contact Zone [5 ,28 ,29 ].
The Bayan Obo deposit comprises West, Main, and East orebodies (Fig. 1 A), with Nb mineralization being particularly enriched in the West orebody and East Contact Zone [5 ].The Main and East orebody each consists of a single lenticular-shaped orebody, whereas the West orebody is composed of several small orebodies (Fig. 1 B, [2 ,5 ]).Within this deposit, monazite and REE fluorocarbonates are the major host of REE, while Fe resource is mainly found in magnetite and hematite, and Nb resource is primarily hosted by aeschynite, ferrocolumbite, fergusonite, pyrochlore, i lmenoruti le, and baotite [15 ].Through extensive field observations, representative samples were collected from outcrops of the open pit and dri l l cores.Based on the mineral occurrences of ferrocolumbites and their paragenetic mineral association of hydrothermal alteration in the host dolomite, three types of ferrocolumbite were identified.
Representative thin sections were selected for further analysis (e.g.SEM and EPMA), with the targeted grains being dri l led to make mounts for SIMS U-Pb dating.Detailed deposit geology, sample location, methods, and data results are presented in the Supplementary Data.

Three-stage Nb mineralization at Bayan Obo
Dating ore minerals directly is one of the best approaches to define the timing of mineralization (e.g.[30 ,31 ]), as this can place mineralizing mechanisms under a robust geological framework.Using SIMS U-Pb dating with a robust matrix-effect correction approach, we have successfully provided accurate U-Pb ages of ferrocolumbite, hence the age of Nb mineralization at Bayan Obo.In comparison to Th-Pb and Sm-Nd isochron ages, the dual decay system of U-Pb provides information on open system behaviors and incorporation of common lead [32 ], which is fundamental for robust interpretation of radiometric dates.For type Ⅰ ferrocolumbite, although many analyses exhibit significant uncertainties arising from low U and high levels of common lead contents, it clearly defines an array on the Wetheri l l Concordia plot (Fig. 4 B) which is a testament to gradual lead loss (Fig. 4 A).The oldest 207 Pb/ 206 Pb age (1366 ± 106 Ma), which experienced the least Pb loss, provides the closest estimate to the true age, and is consistent with the upper intercept age of 1312 ± 47 Ma within uncertainties (Fig. 4 B).Therefore, the firststage Nb mineralization was formed at ∼1.3 Ga.The exact timing of isotopic disturbances cannot be precisely defined due to considerable uncertainties in the lower intercept, but it is likely related to the second and third stage of Nb mineralization as discussed below.For type Ⅱ ferrocolumbite, despite the fact that dates of low-U samples yield dates with significant uncertainties, high-U samples are more precise and are less impacted by common lead.High U samples give a weighted average 206 Pb/ 238 U age of 437 ± 7 Ma ( n = 29, Fig. 4 D), which is consistent with the lower intercept age of 438 ± 7 Ma defined by all spots (both high-U and low-U, n = 93, Fig. 4 C).This 0.44 Ga age is supported by a Th-Pb isochron age of 438 ± 25 Ma [5 ] for aeschynite from the veintype ores that cut through the orebodies and H8 dolomite.
For type Ⅲ ferrocolumbite, the lower intercept age of 268 ± 5 Ma is in good agreement with the weighted average 206 Pb/ 238 U age of 270 ± 4 Ma (Fig. 2 E and F), which clearly suggests a third stage of Nb mineralization at ∼0.27 Ga.This is supported by an aeschynite Th-Pb isochron age of ∼273 Ma [2 ].

Genesis of Nb mineralization at Bayan Obo
Petrographic examination reveals that the ∼1.3 Ga ferrocolumbites are closely associated with hydrothermal minerals [33 ] such as apatite, biotite, chlorite, monazite, pyrite, and minor molybdenite (Fig. 2 A-C).Hence we tentatively link the ∼1.3 Ga ferrocolumbites to hydrothermal fluids.We emphasize that their coexistence with hydrothermal minerals can also be explained by post-formation disturbance, as evidenced by their dissolution texture (Fig. 2 B, C) and lead-loss nature (Fig. 4 A).A Mesoproterozoic carbonatite magma intrusion is further supported by the carbonatite dykes near the mining area [8 ], which has been proposed as a major driver for the extensive fluorine and fenite alteration around the carbonatite dykes [34 ].Given the considerable carry capacity of carbonatite magma for Nb [35 ], the first stage Nb metal could be sourced from Mesoproterozoic carbonatite magma.
The ∼0.44 Ga ferrocolumbites coexist with minerals typical of hydrothermal metasomatism in carbonatite systems [5 ,14 ], such as Sr-and Ba-rich minerals (norsethite, strontianite, barytocalcite, and baryte) and alkaline minerals (biotite and riebeckite, Fig. 2 D-F ), hence the second-stage Nb mineralization was suggested as hydrothermal in origin.Notably, the ∼0.44 Ga ferrocolumbites show distinct compositional differences from the ∼1.3 Ga ferrocolumbites, with lower TiO 2 and MgO contents, and higher MnO contents (Fig. 3 ).Thus, the hydrothermal fluids responsible for second-stage Nb mineralization likely are richer in Sr, Ba, Mn, and alkali compared to that of the ∼1.3 Ga Nb mineralization.The ∼0.44 Ga Nb mineralization is coeval with the early Paleozoic hydrothermal veins that cut through the orebodies and H8 dolomite [25 ,26 ],  linking it closely to the early Paleozoic hydrothermal activity.These hydrothermal fluids are proposed to be released from the subducting slab [5 ,36 ] or originate from the remelting of Mesoproterozoic carbonatite induced by heat generated through Paleozoic plate subduction [37 ].However, ongoing debates persist regarding the subduction dynamics, with some studies proposing southward subduction of the Paleo-Asian Oceanic (PAO) plate towards the North China Craton (NCC) [38 ], while others suggest a northward subduction of South Bainaimiao Ocean, a branch of the PAO located to the north of the NCC [39 ].Additionally, there is speculation that the hydrothermal fluids might originate from an alkaline-carbonatite suite that does not crop out on a plutonic scale in the Bayan Obo area [2 ].These differing opinions emphasize the urgent need for further research to elucidate the mechanisms of early Paleozoic hydrothermal activity and specific processes of Nb enrichment and mineralization.The ∼0.27 Ga ferrocolumbites are intergrown w ith aeschynite w ithin biotite veins which cut through dolomite.They also host inclusions of biotite, pyrite, and apatite (Fig. 2 G-I), hence are interpreted as hydrothermal in origin.The thirdstage Nb mineralization is contemporaneous with Permian granites [28 ].The significantly lower Nb content (16-19 ppm, [40 ]) of Permian granites compared to carbonatite dikes implies a limited contribution of Nb resource from the granites.However, the similar composition of the ∼0.27Ga and ∼1.3 Ga ferrocolumbites (Fig. 3 ), and their close proximity in space ( ∼50 m apart; Fig. 1 B) suggest that the ∼0.27Ga Nb mineralization likely resulted from the reactivation of the ∼1.3 Ga Nb mineralization, facilitated by the Permian granite intrusion.This highlights the important role of granite emplace in the formation of high grade Nb mineralization, which challenges the traditional model that the Permian granites do not contribute metal endowment at Bayan Obo [41 ].While the known hydrothermal and metamorphic effects of Permian granites primarily influence the eastern and southern sides of the H8 dolomite (e.g.[5 ]), the finding of ∼0.27 Ga ferrocolumbite in the West orebody suggests a more extensive impact.This new finding could offer indicative guidance for Nb resource exploration and extraction efforts in the Bayan Obo deposit.
Experimental studies suggest that Nb can be mobi le in al kalic and F-rich hydrothermal systems [42 ].Moreover, the major mechanisms driving Nb enrichment and mineralization involve fluorination and alkaline metasomatism [15 ].Thus, giving the occurrence of alkaline minerals such as biotite and riebeckite, we emphasize the significance of alkaline fluids in all three stages of Nb mineralization.

Implications for REE and Fe mineralization at Bayan Obo
Previous studies have yielded a wide range of radiometric dates using the Sm-Nd and Th-Pb systems for REE-bearing minerals ( ∼1.4-0.26Ga, e.g.[9 ]) at Bayan Obo.The large range of Th-Pb dates has been interpreted as a continuous and prolonged REE mineralization event [10 ], or multiple stages of REE mineralization [43 ], or even the modifications of existing REE mineralization by later thermal-hydrothermal events [9 ].These dates carry considerable uncertainties arising from unaccounted common lead and potential open system behaviors, which are difficult to evaluate by Th-Pb dating using LA-ICP-MS.Combined with detailed petrographic observation and the robust U-Pb system, our study overcame the aforementioned challenges and established a threestage model for Nb mineralization.Given the close spatial association between REE and Nb minerals (Fig. 2 ), it is possible that REE mineralization may have also formed through multistage processes [43 ].

METHODS
Representative thin sections of all samples were coated with carbon for backscattered electron (BSE) imaging by TESCAN integrated mineral analyzer (TIMA) and major element analysis by electron probe microanalyzer (EPMA).After that, suitable regions ∼5 mm in diameter were dri l led out using a micro-dri l l.The dri l led chips as well as the corresponding ferrocolumbite standards were mounted in epoxy mounts for SIMS U-Pb dating.Detailed methods of TIMA mineral mapping and EPMA major element analysis are presented in the Supplementary Data.

SIMS ferrocolumbite U-Pb dating
The SIMS U-Pb analyses were performed using a Cameca IMS-1280HR SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS).The analytical procedure for ferrocolumbite mineral dating is similar to that developed by [18 ], only a brief summary is described here.The O 2 − primary ion beam was accelerated at ∼13 kV, with an intensity of ∼6 nA.The ellipsoidal spot is about 10 × 15 μm in size.The 93 Nb 2 16 O + peak is used as a reference peak for centering the secondary ion beam, energy, and mass adjustments.
A mass resolution of ∼13 0 0 0 (defined at 50% peak height) was used to separate isobaric interferences on the 204 Pb isotope.A single electron multiplier was used in ion-counting mode to measure secondaryion beam intensities by a peak jumping sequence, including isotopes of 93 Nb 2 16 O + , Pb + , Th + , U + , UO + , and 238 U 16 O 2 + .Each measurement consisted of 7 cycles, and the total analysis time of a single spot was ∼16 minutes.
To estimate the Pb/U ages of the ferrocolumbite samples in the absence of a matrix-matched standard, the matrix-effect correction strategy recommended by [18 ] was applied.First, two columbite-tantalite reference materials (NP-02 and ZTA01) of variable Nb/Ta chemical composition have been used as standards.The recommended ages of NP-02 and ZTA01 are 380.3 ± 2.4 Ma [18 ] and 264 Ma [44 ], respectively. 206Pb/ 238 U calibration was done based on the linear relationship between U 16 O + / 238 U + and 206 Pb + / 238 U + ratios.Then, the Nb/Ta chemical composition of the ferrocolumbite samples and the standards were measured by EPMA.Based on the linear correlation between Nb/Ta chemical composition and SIMS age bias, the SIMS matrix-effect can be properly corrected.A long-term uncertainty of 1.5% (1 RSD) for 206 Pb/ 238 U measurements was propagated to the unknowns.
To monitor the precision and accuracy of SIMS U-Pb ferrocolumbite in this study, two in-house columbite standards LCT01 and LCT02 were alternately analyzed as an unknown together with other unknown columbite samples.The independent 207

Figure 1 .
Figure 1.(A) Geological map of the Bayan Obo deposit and surrounding area.(B) Simplified geological map of the West orebody.The green star shows the locations of studied samples.The geological map was modified after [48 ,49 ].

Figure 2 .
Figure 2. TEMSCAN TIMA mineral mapping and backscattered electron images of ferrocolumbite samples from the West orebody, Bayan Obo.(A-C) Images of the sample with type I ferrocolumbite.The ferrocolumbite coexists with and includes hydrothermal minerals.(D-F) Images of the sample with type II ferrocolumbite.The ferrocolumbite has inclusions of baryte, monazite, biotite, riebeckite and strontianite.(G-I) Images of the sample with type III ferrocolumbite.Ferrocolumbite is associated with aeschynite, biotite, apatite, monazite, pyrite, and molybdenite.

2 Figure 3 .
Figure 3. Ternary plot displaying the molar fractions of MnO, MgO, and TiO 2 in three types of ferrocolumbite.Three types of ferrocolumbite are discerned through their mineral occurrences and paragenetic mineral associations of hydrothermal alteration in the hosted dolomite from the West orebody, Bayan Obo.

Figure 4 .
Figure 4. B) 207 Pb/ 206 Pb ages and Wetherill Concordia plot of type I ferrocolumbite, analyses in (B), represented by dull lilac ellipses, are excluded from (A) due to their unusually large 207 Pb/ 206 Pb age uncertainties.(C, D) Tera-Wasserburg plot and weighted average 206 Pb/ 238 U age of type II ferrocolumbite.(E, F) Tera-Wasserburg plot and weighted average 206 Pb/ 238 U age of type III ferrocolumbite.MSWD denotes mean square of weighted deviates.
Pb/ 206 Pb ages of LCT01 and LCT02 are weighted at 1802 ± 5 Ma and 919 ± 4 Ma, respectively.With the above-mentioned calibration procedure, LCT01 and LCT02 yield weighted average 206 Pb/ 238 U ages of 1808 ± 19 Ma and 918 ± 6 Ma, respectively, which are identical within error with their values (see Set S2).The results of in-house columbite standards indicate that our SIMS U-Pb columbite dating method is accurate.